Over the next few years the Human Genome Project will progress from mostly a large scale sequencing effort into one of functional genome characterization, i.e. identifying and characterizing expressed sequences and elucidating structure/function of the proteins derived from translated sequences. The characterization of the expressed sequences has primarily relied upon sequence homology with well-characterized genes (usually to common motifs of gene families or to genes from other organisms), complementation analysis and biochemical analysis of recombinant proteins. However, these efforts have been limited in their capacity to analyze and functionally characterize large numbers of genes. In order to human genes involved in DNA repair and chromosome metabolism, we have developed genetic screens utilizing both the yeast Saccharomyces cerevisiae and E. coli for the isolation of human genes that may play a role in these processes. Specifically, we have screened for human genes that prevent the growth of a yeast pol3 mutant (the gene encoding DNA polymerase d) and a checkpoint mutant RAD9 as well as genes that induce the E. coli SOS when expressed. This approach is based upon the idea that some human genes when expressed in yeast can have a dominant-negative effect on specific endpoints and/or induce the SOS response in E. coli. These genes may affect analogous genetic endpoints in humans. In a series of parallel experiments, we have begun to screen for yeast genes that can induce the E. coli SOS regulon in order to identify genes that can affect chromosome metabolism. The functional characterization of the yeast genome for DNA metabolic genes is particularly relevant given that the sequence of the yeast genome is now complete.